1. Field of the Invention
The present invention relates to a process for carrying out gas-liquid countercurrent processing, especially in the context of petroleum refining and organic chemical industries. In particular, the present invention relates to an improved process for carrying out a countercurrent contacting catalytic reaction in a gas-liquid-solid triphasic system.
2. Description of the Related Art
In the petroleum refining and the organic chemical industries, more and more attention is being paid to the development of gas-liquid countercurrent reactors. The gas-liquid countercurrent reactors achieve increased reaction efficiency between the gas and the liquid phases, and thus require a reduced amount of catalyst, leading to a lowered production cost. Additionally, the gas-liquid countercurrent reactors make it possible for endothermic or exothermic reactions to proceed under quasi-isothermal conditions, thus reducing the operation cost and increasing the economic benefit of the plants.
Besides, as stricter and stricter quality standards of the oil products are adopted throughout the world, the hydrocarbon hydrotreating processes tend to be carried out in the gas-liquid countercurrent reactors. Most of the hydrocarbon hydrotreating processes of the prior art are carried out in fixed-bed reactors, in which hydrogen and the hydrocarbon feedstock flow concurrently downward, contact the catalyst bed to effect catalytic reactions. However, hydrogen sulfide, ammonia and small hydrocarbon molecules resulting from the reactions inhibit further reactions in the process, such as desulfurization, denitrogenation, dearomatization and hydrocracking, giving rise to reduced reaction rates. In particular, the dearomatization reaction is drastically inhibited, for it is a reverse reaction whose reaction extent is proportional to the hydrogen partial pressure but inversely proportional to the reaction temperature, and as such proceeds at a decreasing reaction rate with the increase of the reaction temperature as a result of the hydrogenation plant being adiabatic. In view of all this, graded hydrogenation and/or gas-liquid countercurrent hydrogenation techniques become a good option for carrying out hydrocarbon hydrogenation, so as to render the latter proceeding at an increased hydrogen partial pressure, with little or even no vacillation of reaction temperature and with instant removal of the H2S and NH3 formed during the reactions.
U.S. Pat. No. 5,985,135 proposes a two-stage hydrogenating process wherein an upflow and a downflow reactors are adopted, with a stripping unit immediately downstream of the first reactor. However, the process falls short of solving the problem of removing harmful gases and requires a high capital cost.
With a view to removing the harmful gases, many refineries are conducting research on processes for carrying out gas-liquid countercurrent hydrogenating. The hydrocarbon hydrotreating reactors currently available adopt a single-function catalyst bed consisting of a single type of catalyst loaded together, with the reaction proceeding in liquid phase. To guarantee the effectiveness factor of the catalyst, the reactor is only loaded with small particles of catalyst. On the other hand, the catalyst is loaded to a relatively big height, and is immersed in liquid; therefore, it must be in solid form to have enough mechanical strength. Indeed, in industrial gas-liquid countercurrent hydrogenating reactors, the catalyst is used as sphere or cylinder with a diameter less than 3 mm, and is loaded at a catalyst bed voidage of 0.35 to 0.45, much lower than the voidage for conventional countercurrent operations (>0.95). However, at such a catalyst bed voidage, the countercurrent reactors tends to cause flooding and an unstable operation of the plant.
U.S. Pat. No. 5,985,131 and U.S. Pat. No. 6,007,787 propose a method for loading catalyst wherein a gas by-pass is installed to avoid flooding. Such a method makes it possible for the countercurrent reactor to hold a reduced volume of liquid, and allows the plant to run at an increased range of gas-liquid ratio. Nonetheless, it is devised without regard to the gas-liquid distribution of the whole reactor, and cannot eliminate flooding in the reactor. Additionally, it gives rise to a lowered utilization of the reactor, and a lowered flexibility in the case of hydrotreating reactor, which is usually bulky. There are processes in the chemical industry and environmental protection section with similar defects.
U.S. Pat. No. 5,183,556 proposes a countercurrent hydrorefining process, employing a currently available plant, wherein the countercurrent reaction section is loaded with a catalyst of a single type, e.g. a catalyst comprising sulfides of non-noble metals or comprising noble metals. This process has the defects of flooding. In addition, it does not apply to drastic desulfurization and dearomatization of the feedstock. As the feedstock reaches greater and greater reaction extent, it undergoes further reactions at a lower and lower rate. In order to meet the product standards, the process requires high activity catalysts, preferably noble metal catalyst in the case of the dearomatization. However, if the process employs high activity catalysts, for example noble metal catalyst in the whole catalyst bed, the catalyst in the upper section of the reactor will be subject to deactivation as a result of the high H2S partial pressure. On the contrary, if the process employs a conventional catalyst of sulfides of non-noble metals, the catalyst in the lower section of the reactor will also be subject to deactivation due to sulfur loss as a result of the low H2S partial pressure, even loses all its activity.
In the gas-liquid countercurrent processing, the gas and liquid phases undergo constant changes in volume and viscosity respectively, which have an important influence on the operating stability of the plant. GB 8618888 relates to a process for synthesizing dialkyl maleate, comprising reacting maleic anhydride and an alkanol to give a monoalkyl maleate, and then reacting monoalkyl maleate with vapour of the alkanol in countercurrent flow in the presence of a catalyst. In the process, an evenly distributed bed of resin catalyst is employed. In the direction of the flow of the liquid phase, the viscous liquid stream of dialkyl maleate has an ever increasing content of dialkyl maleate; the gas phase becomes smaller and smaller in volume owing to its consumption during the reactions and thus cannot remove all the water formed during the reactions. In the end, the reactor has higher flow rates of the gas phase and the more viscous liquid phase in the lower section, where flooding tends to occur, and lower flow rates of the gas phase and the less viscous liquid phase in the upper section, where flooding rarely occurs. The process employs an evenly distributed catalyst bed without regard to the changing gas-liquid ratios at different sections of the reactors, and thus fails to improve the comprehensive operating flexibility and stability of the reactor. There are other similar processes with such defects.